1. Field of the Invention
This invention relates to illuminated electrooptic display devices useful for horological purposes and, more particularly, to such display devices which are operative in the reflective and transmissive modes and include a so-called transflector for this purpose. In particular, the invention involves an improved transflector which enhances the viewability of illuminated liquid crystal display devices.
2. Description of the Prior Art
Various types of electrooptic displays have been employed for electronic wristwatches, clocks and other devices. One type of display is the field effect, twisted nematic liquid crystal display, the operation and construction of which are well known and described, for example, in the Fergason, U.S. Pat. No. 3,731,968. It is also well known that displays can operate both in the reflective and transmissive modes; ie., using either reflected ambient light for daytime viewing or backlighting with one or more actuable light sources behind the display for nighttime viewing. In this regard, see the Girard U.S. Pat. No. 3,712,047; Luft U.S. Pat. No. 3,799,647 and Richardson U.S. Pat. No. 3,864,905. The latter patent employs an electrical lamp at the center of an edge of the display and means for dispersing light rays or specular light.
Also known are means for illuminating clock dials using a light source with means to first disperse the specular light rays to the desired location and then to diffuse, ie, scatter, the light in a particular area. Exemplary are U.S. Pat. No. 2,916,871 to Wegner, U.S. Pat. No. 3,514,940 to Kern et al, and U.S. Pat. No. 3,574,993 to Black. The latter patent shows that it is known to employ a diffusion plate of transparent plastic material for carrying light rays, the plate having a reflector adjacent one surface and a roughened contour on the other surface to scatter light. British Pat. No. 1,417,958 discloses a similar light conductor which is a transluscent member, for example, frosted glass.
A particular type of liquid crystal display includes a liquid crystal cell having image-forming electrodes, upper and lower polarizers on opposite sides of the cell and a so-called transflector behind the lower polarizer. A light source typically is positioned behind or at the side of the transflector for directing light therein. The transflector functions essentially as a oneway mirror so that the display can operate in the reflective mode in ambient light and in the transmissive mode at night with the aid of backlighting by the light source. In daylight, ambient light enters the display and passes through the liquid crystal cell, where it may be optically altered, and then is reflected from the transflector back toward the observer. At night, the transflector receives light from the light source and transmits it upwardly toward the observer by light scattering, diffusion, and reflection within the transflector. As a result of the light diffusion effected by the transflector, the display characters are more or less uniformly illuminated. A typical transflector may comprise silvered glass or glass with a gray filter. However, other materials which are transluscent to light have also been widely used. Exemplary of these are styrofoam of white, gray or silver hue, polystyrene, and polypropylene in thin layers or sheets. Although satisfactory back lighting or liquid crystal displays has been achieved with the aid of the transflector, it has not been obtained without some sacrifice in the daylight viewability of the display. More specifically, the quality of the daylight image has been adversely affected by the transflector in that a reduction in the contrast between the image and background, especially a dark image on a light background has been observed. Of course, any reduction in the contrast ratio of a liquid crystal display adversely affects not only the aesthetic appeal to the observer but also its utility. To this end, a concentrated effort has been made to improve the quality of daylight viewability of back-lighted liquid crystal displays employing transflectors.